Method for operating an internal combustion engine with direct fuel injection

ABSTRACT

A method of operating an internal combustion engine with an injection device, wherein the method includes the steps of feeding combustion air to a combustion chamber via an inlet port, injecting fuel into the combustion chamber using a fuel nozzle arranged in the combustion chamber, igniting a formed fuel/air mixture at a certain ignition point using a spark plug arranged in the combustion chamber, and, during the starting of the internal combustion engine, selecting a high-pressure or a low pressure start as a function of a minimum fuel pressure built up in the injection device within a defined number of cycles. The minimum fuel pressure and the number of cycles are selected as a function of a combustion-chamber temperature. The injection of the fuel into the combustion chamber takes place in a timed sequence during the starting operation.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a method of operating an internal combustionengine with direct fuel injection.

During a starting phase of a spark-ignition internal combustion enginewith direct fuel injection, large quantities of unburned fuel areproduced, in particular during a cold start, since thecombustion-chamber temperature of the internal combustion engine is lowduring a cold starting phase. Therefore adequate vaporization of theinjected fuel cannot take place. On account of the low cylinder-walltemperature, some fuel proportions accumulate on the cylinder wall ofthe internal combustion engine and do not participate in the combustion.

U.S. Pat. No. 5,482,017 discloses a method of operating a spark-ignitioninternal combustion engine with direct fuel injection. In this method,during a starting phase, a fuel quantity which is greater than therequisite fuel quantity is injected during the first combustion cycle inorder to compensate for the fuel proportions which have accumulated onthe cylinder wall. As a result, reliable ignition of the fuel/airmixture formed is achieved. During the subsequent combustion cycles, alean fuel/air mixture is formed in the combustion chamber and is ignitedearlier in order to increase the combustion-chamber temperature or thecylinder-wall temperature. In order to heat a downstream exhaust-gastreatment device, the fuel quantity, during the subsequent combustioncycles, is then divided into an early and a late injection and a laterignition of the fuel/air mixture formed is carried out.

Patent Document WO 99/67526 discloses a method of operating aspark-ignition internal combustion engine with direct fuel injection. Inthis method, the pressure of the fuel injected into the combustionchamber is determined, so that the fuel is injected during the inductionphase of the internal combustion engine when the fuel pressure is lowerthan a predetermined minimum pressure. In this case, a pressure sensoris provided, by means of which the pressure of the fuel injected intothe combustion chamber is measured before the fuel enters the combustionchamber. The internal combustion engine is run in a stratified operationduring the starting phase if a minimum pressure is exceeded and is runin a homogenous operation if the pressure drops below the minimumpressure.

DE 198 232 80 C1 discloses a method of operating a direct-injectioninternal combustion engine during the starting phase, in which method,as a function of a coolant temperature, the operation is changed overbetween a low-pressure start with a homogenous mixture and ahigh-pressure start with a stratified mixture. In this case, ahigh-pressure start of the internal combustion engine is initiated ifthe pressure in a fuel pressure accumulator has exceeded a predeterminedthreshold value. Here, the threshold value is stored in a characteristicmap of the control device as a function of a coolant temperature.

With the methods described above, no optimum combustion is achievedduring the starting operation of a spark-ignition internal combustionengine with direct fuel injection, since an operating behavior of theinternal combustion engine during the starting phase without sparkfailure cannot be ensured.

Against this background, the object of the invention is to design theinjection process during the starting phase in such a way that wettingof the combustion-chamber wall with fuel during the injection process isreduced and an ignitable mixture cloud is formed in the combustionchamber of the internal combustion engine in the vicinity of an ignitionsource.

This object is achieved according to the invention of operating aninternal combustion engine with an injection device, wherein the methodincludes the steps of feeding combustion air to a combustion chamber viaan inlet port, injecting fuel into the combustion chamber using a fuelnozzle arranged in the combustion chamber, igniting a formed fuel/airmixture at a certain ignition point using a spark plug arranged in thecombustion chamber, and, during the starting of the internal combustionengine, selecting a high-pressure or a low-pressure start as a functionof a minimum fuel pressure built up in the injection device within adefined number of cycles. The minimum fuel pressure and the number ofcycles are selected as a function of a combustion-chamber temperature.The injection of the fuel into the combustion chamber takes place in atimed sequence during the starting operation.

The method according to the invention is characterized in that theminimum fuel pressure and the number of cycles during the starting ofthe internal combustion engine are selected as a function of acombustion-chamber temperature, the injection of the fuel into thecombustion chamber preferably taking place in a timed sequence duringthe starting operation. Due to the timed sequence of the injected fuelquantity during a cycle, wetting of the combustion-chamber wall with theinjected fuel is minimized, as a result of which the emissions, inparticular the unburned fuel proportions, are reduced during thestarting phase. Since a high-pressure start is not initiated untilsufficient fuel pressure is present, improved atomization of theinjected fuel is ensured. Furthermore, a pressure drop in the injectiondevice during the high-pressure start on account of the high demand forfuel during the starting of the internal combustion engine is preventedby the timed sequence of the injected fuel quantity.

In a configuration of the method according to the invention, during thestarting operation, the total fuel quantity injected is introduced intothe combustion chamber in up to three partial quantities, i.e. the totalfuel quantity can be optionally introduced into the combustion chamberin the form of one, two or three partial quantities. The timed sequenceof the injected fuel quantity prevents combustion air from penetratinginto the fuel injector or into the fuel distribution line, for example acommon rail, during a low-pressure start on account of the low fuelsystem pressure. The injection during the compression stroke thereforetakes place early enough irrespective of the injection strategy, so thatthe compression pressure does not exceed the fuel injection pressure.During the high-pressure start, due to the timed sequence of theinjected fuel quantity, a homogeneous start, a mixed form consisting ofa homogenous and a stratified start, or a pure stratified start can beachieved by varying the injection times.

According to a further configuration of the invention, during alow-pressure start, the up to three partial quantities are injected intothe combustion chamber before the ignition point, and, during ahigh-pressure start, the up to two partial quantities are injected intothe combustion chamber before the ignition point and one partialquantity is injected into the combustion chamber after the ignitionpoint. Since, during the low-pressure start, the injection of thepartial quantities—there may be one, two or three partial quantities—iscompleted before the ignition point, high HC emissions are prevented andreliable combustion of the total fuel quantity is ensured.

In a further configuration of the invention, the ignition point duringthe starting operation is regulated as a function of thecombustion-chamber temperature and a difference between an actual speedand an idling speed. The relationship between the ignition point and thecombustion-chamber temperature ensures that the combustion-chambertemperature is increased during the first cycles.

According to a further configuration of the invention, during ahigh-pressure start, if the fuel pressure drops below a defined minimumpressure in the injection device, the operation is changed over to thelow-pressure start. This creates defined pressure conditions whichpermit controlled metering of fuel. This ensures that no wall wettingtakes place due to the late injection of fuel.

In a further configuration of the invention, the combustion-chambertemperature is recorded by means of a temperature-measuring device atthe combustion chamber or with reference to a coolant temperature of theinternal combustion engine. A temperature probe is preferably attachedto the cylinder wall in the region of the combustion chamber.Alternatively, a coolant temperature serves as a reference quantity fordetermining the combustion-chamber temperature, it being possible, inaddition to or as an alternative to the coolant temperature, for thetemperature of the drawn-in air mass to be used as a further referencequantity for determining the combustion-chamber temperature.

According to a further configuration of the invention, a low-pressurestart is effected at a coolant temperature of less than −15° C. orgreater than 90° C., a high-pressure start being effected at a minimumfuel pressure, built up in the injection device, of at least 10 bar andat a coolant temperature of between −15° C. and 90° C. At a fuelpressure of at least 10 bar, good spray quality of the injected fuel isensured and vaporization of the injected fuel is achieved without fuelaccumulating on the wall. In this case, a low-temperature start ispreferred at temperatures greater than 90° C., since, due to hightemperatures in the combustion chamber, sufficient fuel pressure oftencannot be built up during a restarting operation of the internalcombustion engine on account of the increasing gap sizes in the fuelpump. The low-pressure start preferably takes place within a temperaturerange of less than −30° C. or greater than 110° C., and thehigh-pressure start preferably takes place between −30° C. and 110° C.

Further features and feature combinations follow from the description.Practical exemplary embodiments of the invention are shown simplified inthe drawings and are described in more detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cylinder cross section of a direct-injection internalcombustion engine with spark ignition,

FIG. 2 shows a schematic diagram of the injection characteristic of theinternal combustion engine according to FIG. 1 during a low-pressurestart, plotted against the crank angle, and

FIG. 3 shows a schematic diagram of the injection characteristic of theinternal combustion engine according to FIG. 1 during a high-pressurestart, plotted against the crank angle.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cylinder 2 of a direct-injection internal combustionengine 1, in which a piston 3, arranged in a longitudinally movablemanner, together with a cylinder head 7 closing the cylinder 2 defines acombustion chamber 4. Arranged in the cylinder head 7 is a fuel injector5, in which fuel is injected in the form of a fuel cone 9 into thecombustion chamber 4 through a nozzle hole 6. In the internal combustionengine 1, during a starting phase, by means of a fuel pressuredetermined in an injection device (not shown), a control device (notshown) determines whether a high-pressure start or a low-pressure startis initiated during the starting of the internal combustion engine 1.When the nozzle hole 6 is opened up, the fuel is injected in a conicalspray 9 into the combustion chamber 4 in such a way that the spark plug11 arranged in the cylinder head 7 is essentially not wetted. In thiscase, the fuel spray 9 is introduced into the combustion chamber with aspray angle α which preferably lies within a range of between 70° and110°. The internal combustion engine 1 shown in FIG. 1 works accordingto the four-stroke principle, the method according to the inventionlikewise being suitable for spark-ignition two-stroke internalcombustion engines with direct injection.

In the first stroke of an operating cycle of the internal combustionengine from FIG. 1 working according to the four-stroke principle,combustion air is fed to the combustion chamber 4 through an inlet port8, the piston 3 moving downward to a bottom dead center BDC. In afurther compression stroke, the piston 3 moves upward from the bottomdead center BDC up to a top ignition dead center IDC. In the region ofthe ignition dead center IDC, the fuel/air mixture formed is ignited bymeans of the spark plug 11, in the course of which the piston 3 expandsin a downward movement to a bottom dead center BDC. In a last stroke,the piston 3 extends in an upward movement up to a top dead center TDCand expels the exhaust gases from the combustion chamber 4.

Depending on a cooling-water temperature measured in a cooling-waterpassage 12, a control device (not shown) establishes whether ahigh-pressure or a low-pressure start is carried out during the startingphase of the internal combustion engine. A high-pressure start isinitiated when a minimum fuel pressure is present in a fuel supply line(not shown), which is designed, for example, as a fuel common rail. Theminimum fuel pressure is established as a function of the cooling-watertemperature determined. This minimum fuel pressure is at least 10 bar.At such a fuel pressure, it is ensured that good atomization quality isachieved. The minimum fuel pressure must in turn be built up after adefined number of combustion cycles. According to the present exemplaryembodiment, a cycle refers to a process from the first to the fourthstroke according to the four-stroke sequence described above. Theminimum fuel pressure is preferably to be built up after approximatelyfour seconds. In this case, the number of combustion cycles isestablished as a function of the cooling-water temperature.

If the fuel pressure does not reach the minimum fuel pressure after adefined number of cycles, a low-pressure start is initiated. Accordingto FIG. 2, the fuel is introduced in up to three partial quantities intothe combustion chamber before the ignition point IP. The division of thefuel quantity permits better distribution of the injected fuel in thecombustion chamber 4 and relieves the fuel system. The first partialquantity FI1 _(LP) is introduced into the combustion chamber 4 duringthe induction phase. The second partial quantity FI2 _(LP) appears in aregion between a final part of the induction phase and an initial partof the compression phase. The third partial quantity FI3 _(LP) thenappears just before the ignition point IP, so that an ignitable mixturecloud is present in the region of the spark plug 11. The injection timesshown in FIG. 2 are illustrated schematically; they can be freelyselected, in which case they are varied as a function of an actual speedand/or of a cooling-water temperature.

The end of the fuel injections FI1 _(LP), FI2 _(LP), FI3 _(LP) isestablished via three characteristic maps. The characteristic maps areeach defined via the cooling-water temperature and the number of cycles.In this case, the fuel injection is configured in such a way thatoverlapping of fuel injections is prevented. It is therefore conceivableto establish the end of the third injection FI3 _(LP) and the twopreceding injections FI1 _(LP) and FI2 _(LP) via delay times ordifferential angles. During the low-pressure start, the injection periodof the last injection FI3 _(LP) is limited on account of the low fuelpressure in order to prevent fuel and air from entering the injector. Inthis case, pressure compensation ensures that the correct injection timeis always set. During the low-pressure start, a pressure regulator inthe injection device is preferably kept open in order to achieve definedpressure conditions in the fuel distribution line of the injectiondevice. The injection times shown in FIG. 2 of the three partialquantities are only shown as an example, the injection times beingfreely selectable and being varied as a function of the fuel pressure inthe injection device. In principle, a low-pressure start takes place ata coolant temperature of less than −15° C. or greater than 90° C.

If the minimum fuel pressure is reached after a defined number ofcycles, a high-pressure start is initiated. When a high-pressure startis initiated, the injection timing is defined via the end of injection.In this way, a direct relationship to the ignition is established andthus simple control of the fuel injection is achieved. According to FIG.3, the injection of the fuel during the high-pressure start is effectedin up to three partial quantities FI1 _(HP), FI2 _(HP) and FI3 _(HP).The first partial quantity FI1 _(HP) is injected during the inductionphase. The division of the fuel mass is established with the aid ofcharacteristic maps which are stored in the control unit and in whichthe end of the first injection FI1 _(HP) is established independently ofthe ignition point IP via the cooling-water temperature, an actual speedand an idling speed. The second fuel injection FI2 _(HP) is effectedduring the compression phase in such a way that the interval between theend of the second fuel injection FI2 _(HP) and the ignition point IP isestablished as a function of the cooling-water temperature, the actualspeed and the idling speed. The third fuel injection FI3 _(HP) iseffected just after the ignition point IP, the third fuel injection FI3_(HP) being established via a delay time between the second and thethird fuel injection FI3 _(HP) in such a way that it participates in thecombustion. It is conceivable for only a single fuel injection to becarried out during the high-pressure start, this single fuel injectionthen advantageously taking place within a range of 40° C.A to 160° C.Abefore IDC.

Accordingly, a combined combustion-chamber charge consisting ofhomogeneous charge and stratified charge is achieved during thehigh-pressure start. The first partial fuel quantity constitutes thehomogeneous proportion and the second and the third partial fuelquantity constitute the stratified proportion. As a result,accumulations of fuel on the wall are minimized and reliable ignition ofthe fuel cloud formed is achieved. It is preferably ensured by means ofpressure compensation in the fuel injection device that the correctinjection time is always selected during the starting phase.

With the described multiple injection during the starting phase, theoperating behavior of the internal combustion engine 1 is improved, inparticular the formation of emissions due to the unburned fuelproportion on account of a low wall temperature being minimized. Theseadvantages are preferably obtained when using a piezoelectric injector,in which short injection times, for example less than 0.25 msec, can beachieved. This makes possible the injection of very small partial fuelquantities into the combustion chamber 4, and the requisite fuelquantity for the starting operation of the internal combustion engine 1is greatly reduced.

During the starting phase, the ignition point IP is controlled as afunction of the cooling-water temperature and the engine speed by meansof the control unit during both the low-pressure and the high-pressurestart. In this case, the ignition point IP is stored in a characteristicmap, in which the ignition point IP is plotted against the cooling-watertemperature, the difference between the actual speed and the idlingspeed. After leaving the starting phase, the ignition point IP isbrought into line with the normal operating state. The ignition pointsapplied in the control unit are stored in separate characteristic mapsfor the low-pressure and the high-pressure start.

In particular, the method according to the invention is suitable for usein direct-injection internal combustion engines with spark ignition, inwhich a spray-directed combustion behavior is present. Injection nozzlesopening outward are used in such internal combustion engines, the fuelin these injection nozzles being injected as a hollow cone into thecombustion chamber. The fuel is preferably introduced into thecombustion chamber as a hollow fuel cone having an angle α of between70° and 100°, so that the hollow fuel cone, during a fuel injection inthe compression stroke, strikes combustion air compressed in thecombustion chamber. As a result, a toroidal swirl is formed in the outerregion or at the margin of the injected hollow fuel cone, whereby anignitable fuel/air mixture is provided in the region of the electrodesof the spark plug. In this case, the spark plug is arranged in such away that the electrodes of the spark plug 11 project into the marginalswirl achieved without being substantially wetted during the fuelinjection; i.e., during light or slight wetting of the electrodes of thespark plug, the largest proportion of the fuel should be vaporized againat the electrodes up to the ignition point.

1.-7. (canceled)
 8. A method of operating an internal combustion enginewith an injection device, comprising: feeding combustion air to acombustion chamber via an inlet port; injecting fuel into the combustionchamber using a fuel nozzle arranged in the combustion chamber; ignitinga formed fuel/air mixture at a certain ignition point using a spark plugarranged in the combustion chamber; and during the starting of theinternal combustion engine, selecting a high-pressure or a low-pressurestart as a function of a minimum fuel pressure built up in the injectiondevice within a defined number of cycles, wherein the minimum fuelpressure and the number of cycles are selected as a function of acombustion-chamber temperature, and wherein the injection of the fuelinto the combustion chamber takes place in a timed sequence during thestarting operation.
 9. The method as claimed in claim 8, comprising,during the starting operation, injecting fuel into the combustionchamber in up to three partial quantities.
 10. The method as claimed inclaim 9, comprising, during a low-pressure start, injecting the up tothree partial quantities into the combustion chamber before the ignitionpoint, and, during a high-pressure start, injecting one or two of the upto three partial quantities into the combustion chamber before theignition point and one partial quantity is injected into the combustionchamber after the ignition point.
 11. The method as claimed in claim 10,comprising determining the ignition point during the starting operationas a function of the combustion-chamber temperature and a differencebetween an actual speed and an idling speed.
 12. The method as claimedin claim 11, comprising, during a high-pressure start, if the fuelpressure drops below a defined minimum pressure in the injection device,changing the operation over to the low-pressure start.
 13. The method asclaimed in claim 12, comprising recording combustion-chamber temperatureusing a temperature-measuring device at the combustion chamber or withreference to a coolant temperature of the internal combustion engine.14. The method as claimed in claim 13, comprising effecting alow-pressure start at a coolant temperature of less than −15° C. orgreater than 90° C., effecting a high pressure start at a minimum fuelpressure, built up in the injection device, of at least 10 bar and at acoolant temperature of between −15° C. and 90° C.
 15. The method asclaimed in claim 1, comprising determining the ignition point during thestarting operation as a function of the combustion-chamber temperatureand a difference between an actual speed and an idling speed.
 16. Themethod as claimed in claim 1, comprising, during a high-pressure start,if the fuel pressure drops below a defined minimum pressure in theinjection device, changing the operation over to the low-pressure start.17. The method as claimed in claim 1, comprising recordingcombustion-chamber temperature using a temperature-measuring device atthe combustion chamber or with reference to a coolant temperature of theinternal combustion engine.
 18. The method as claimed in claim 1,comprising effecting a low-pressure start at a coolant temperature ofless than −15° C. or greater than 90° C., effecting a high pressurestart at a minimum fuel pressure, built up in the injection device, ofat least 10 bar and at a coolant temperature of between −15° C. and 90°C.